The present invention relates to a lead frame, particularly a lead frame which uses, as a base material, a base having a metal such as a copper-type metal as the main component and which has bumps made of a metal on the surfaces of inner lead tip portions. The invention also relates to a manufacturing method of such a lead frame, a semiconductor device using such a lead frame, an assembling method of the semiconductor device, and an electronic apparatus using the semiconductor device.
Lead frames are indispensable for the techniques of forming leads of respective electrodes of a semiconductor chip, and commonly used in mounting semiconductor chips. Lead frames are generally made of a copper-type material, and bumps are formed on the surfaces of respective inner lead tip portions so as to be bonded to respective electrode pads on a semiconductor chip. Although previously such bumps were using aluminum, in recent years techniques of forming bumps using gold have been developed. The reasons for forming bumps made of gold are that good bonding performance is obtained and that gang bonding is enabled which provides high production efficiency than single point bonding.
In particular, the fact that gang bonding is enabled is very important for the following reason. Electrode pads of a semiconductor chip are made of aluminum. According to the current technology, single point bonding should be used to bond leads of the previous type having aluminum bumps to such aluminum pads. Where bumps are made of gold, gang bonding can be performed and hence the bonding efficiency can greatly be increased.
FIG. 1 illustrates bonding in a case where bumps are made of gold. Respective inner leads made of copper, for instance, have bumps (having a three-layer structure of gold, tin, and aluminum that are provided in this order from the surface) at the tip portions. The bumps are positioned above respective electrode pads (having a three-layer structure of gold, tin, and aluminum that are provided in this order from the surface) of a semiconductor chip and then bonded thereto by ultrasonic bonding by using a bonding tool.
The conventional techniques of forming bumps made of gold will be described below. In a first technique, the bump-forming side of each inner lead made of copper or the like is selectively half-etched so that a bump-forming portion is protruded. Then, the entire lead surface is plated with gold (a nickel plating layer is formed as an undercoat). In a second technique, gold to constitute a bump is transferred to each inner lead surface by a transfer method. In a third method, gold is evaporated on a bump-forming portion of each inner lead. In a fourth method, the entire surface of each inner lead is plated with a gold film with a nickel plating layer formed as an undercoat.
Each of the above conventional techniques of forming bumps made of gold which provide good bonding performance and enable gang bonding has the following problems. The first technique, in which the bump-forming side of each inner lead is selectively half-etched so that a bump-forming portion is protruded and then the entire lead surface is plated with gold, has a problem that because copper that constitutes leads is hard, electrode pads of an IC chip are prone to cracking. Since a nickel plating layer needs to be formed as an undercoat of a gold plating layer, the number of steps is increased. Further, where gold is formed on the entire lead surface, a large amount of gold is used, to increase the material cost, that is, the overall cost.
The second technique, in which gold to constitute a bump is transferred to each inner lead surface by a transfer method, has a problem that it is prone to positional deviations because of low accuracy of the transfer position control.
The third technique, in which gold is evaporated on a bump-forming portion of each inner lead, has problems that it requires a large investment in equipment because evaporation equipment is expensive, that the productivity is low because gold evaporation takes long time, and that the adhesion between a gold evaporation film and a copper lead is insufficient.
In the fourth technique, in which the entire surface of each inner lead is plated with a gold film with a nickel plating layer formed as an undercoat, has a problem that the number of steps is increased because the nickel plating layer needs to be formed as the undercoat of the gold plating layer. Further, where gold is formed on the entire lead surface, a large amount of gold is used, to increase the material cost. In particular, the gold plating film needs to be formed so as to be thick enough to secure good bonding performance. Forming so thick a gold plating film consumes a very large amount of gold, to cause a non-negligible cost increase.